The invention relates to a new steel material which is manufactured in a non-powder metallurgical way, comprising manufacturing of ingots or castings from a melt. The steel material consists of an alloy, which besides iron and carbon, contains chromium, vanadium, and molybdenum as its substantial alloying elements in amounts which are chosen and balanced in such a way that the steel after hardening and tempering has a hardness and a microstructure which makes the material suitable in the first place for cold work tools but also for other applications where high requirements are raised on wear resistance and comparatively good toughness, such as materials for shaping or working ceramic masses, e.g. for tools to be used in the brick-making industry. The invention also relates to the use of the steel material and to a method for the manufacturing of the material, including the method for the heat treatment of the material.
In the first place tool steels containing more than 10% chromium, which are manufactured conventionally, are used as materials for cold work tools, on which very high requirements are raised as far as hardness and wear resistance are concerned. The standardised steels AISI D2, D6, and D7, which today are used for abrasive cold work applications, are typical examples of this type of steels. The nominal compositions of these known steels are stated in Table 1.
Like all ledeburitic steels, steels of the above mentioned type solidify through the precipitation of austenite, whereafter M7C3-carbides are formed in the regions of residual liquid phase. This gives a material which does not satisfy high requirements on some product features which are of significant importance for cold work steels, namely good abrasive wear resistance in combination with good toughness. It is also a drawback with these conventional ledeburitic tool steels that they have a rather bad hot-workability.
As materials for cold work steels there are also used tool steels with high contents of vanadium, which are manufactured powder metallurgically. Those steels which are known by their trade names Vanadis(copyright)4 and Vanadis(copyright)10 are examples of this type of steels. The nominal compositions of these steels are stated in Table 2.
The above, powder metallurgically manufactured steels offer extremely good combinations of wear resistance and toughness but are expensive to manufacture.
It is a purpose of the invention to provide a new steel material of steel alloy which can be manufactured in a conventional way through the manufacturing of a melt, from which there are cast ingots, which can be hot-worked to the shape of bars, plates, etc, of which there can be manufactured tools or other articles, which can be heat treated for the achievement of a final product having the desired combination of features. The conventional ingot manufacturing can be completed through some subsequent melt-metallurgical process-step, such as e.g. electro-slag-refining (ESR) or, as an alternative process, the building up of ingots of molten metal drops which are caused to solidify, such as according to the process which is known by the name of Osprey.
The field of use of the material of the invention may include anything from wear parts, e.g. within mining industry, to tools within the field of conventional cold work for the manufacturing of tools for blanking and forming, cold extrusion tooling, powder pressing, deep drawing etc, and tools or machine components for forming or working of ceramic masses, e.g. in the brick making industry. In connection herewith it is a particular objective of the invention to provide a material which has a better combination of wear resistance and toughness than conventional ledaburitic cold work steels of type AISI D2, D6, or D7.
Further it is an object of the invention to provide a material of an alloy which has a better hot workability than the said conventional ledaburitic cold work steels, wherein the yield in production in forging shops and rolling mills can be improved and hence also the production economy.
It is also a purpose of the invention to provide a material having good heat treatment properties. Thus it shall be possible to hardened the steel from austenitising temperatures below 1200xc2x0 C., preferably from temperatures between 900 and 1150xc2x0 C., typically from 950 to 1100xc2x0 C. and the steel shall have a good hardenability; a good dimensional stability on heat treatments; and attain a hardness of 55-66 HRC, preferably 60-66 HRC, through secondary hardening.
An acceptable cutability and an acceptable grindability are other desirable features.
These and other aims can be achieved therein that the invention is characterised by what is stated in the appending, independent patent claims.
FIG. 1 illustrates a typical constitutional diagram of an alloy having vanadium, carbon, and molybdenum contents according to the invention and varying chromium contents. The diagram shows the phases in a state of equilibrium at different temperatures. When an ingot or a casting is caused to solidify slowly, the alloy will solidify through a primary precipitation of hard particles of MX-type in molten phase, where M is V and/or Nb, but preferably V, and X is C and/or N, but preferably C. The remaining, residual melt has a comparatively low content of alloying elements and will solidify to form austenite and MX (xcex3+MX region in the phase diagram). During continued cooling, the xcex3+MX+M7C3-region is passed rather quickly, in which region a smaller amount of carbides of M7C3-type can be precipitated, where M substantially is chromium.
Thus it is typical for the material of the invention that its micro-structure at the temperature 1100xc2x0 C. in the state of equilibrium consists of austenite in molten phase, and hard particles of MX-type precipitated in the liquid phase, said M being V and/or Nb, but preferably V, and X is C and N, and also, possibly, a smaller amount of secondarily precipitated hard particles, normally max 2%, preferably max 1 vol-%, in the first place M7C3-carbides, in which M substantially is Cr.
The solidified structure of conventional ledaburitic cold work steel, which typically is lamellar, thus is replaced by an even distribution of hard components of MX-type, more than 50 vol-% of which having sizes within the range 3-20 xcexcm and, typically a more or less round or elongated, rounded shape and possibly with a smaller amount of lamellar, solidified structure consisting of M7C3-carbides. After hot-working there is achieved a pronouncedly homogenous and finely dispersed carbide distribution, which is believed to be the main reason why the steel achieves a better hot workability than conventional ledaburitic cold work steels which are manufactured in a non-powder metallurgical way.
In connection with heat treatment comprising hardening and tempering, the material is heated to the xcex3+MX-region of the phase diagram, wherein any existing M7C3-carbides, are dissolved and there is again achieved a structure consisting of austenite and hard particles of MX-type distributed in the austenite. At rapid cooling to ambient temperature, the austenite is transformed to martensite. The xcex3+M+M7C3-region is passed comparatively quickly, which suppresses the formation of M7C3-carbides. Therefor it is also typical for the steel material of the invention that it at room temperature has a microstructure consisting of a matrix which substantially consists of martensite and in this matrix 10-40 vol-% and at some preferred embodiments of the invention, e.g. steels for cold work tools, more particularly 10-25 vol-%, and at some other preferred embodiments of the invention, such as for tools or machine components for the working of ceramic masses, e.g. within the brick-making industry, most conveniently 20-40 vol-% of said primary hard particles of MX-type which are precipitated in liquid phase, said hard particles typically having a rounded shape. Further, there may exist secondarily precipitated hard particles of sub-microscopic size. Because of the small size of the secondarily precipitated particles, it is difficult to determine their chemical composition and also the amount of them without access of very advanced equipment. However, it can be presupposed that such products exist to some extent and then substantially in the form of MC-carbides and M7C3-carbides, in which M is substantially vanadium and chromium, respectively. After hardening and tempering, the material of the invention has a hardness between 55 and 66 HRC, the said microstructure and hardness being obtainable by heating the material to a temperature between 900 and 1150xc2x0 C., through-heating the material at said temperature for a period of time of 15 min-2 h, cooling the material to room temperature and tempering it one or several times at a temperature of 150-650xc2x0 C.
As far as the individual alloy elements and their interaction are concerned, the following apply.
Vanadium, carbon, and nitrogen shall exist in a sufficient amount in order that the material shall be able to contain 10-40 vol-%, and at some preferred embodiments of the invention, e.g. steels for hot worked tools, more particularly 10-25 vol-%, and at some other preferred embodiments of the invention, such as for tools or machine components for working ceramic masses, e.g. in the brick manufacturing industry, more particularly 20-40 vol-% hard particles of MX-type, and the matrix also contain 0.6-0.8% carbon in solid solution, wherein the fact that some carbon and nitrogen can be bound in the form of said, secondarily precipitated hard particles, in the first place M7C3-carbides, also shall be considered. It shall be mentioned that nitrogen normally does not contribute to any substantial degree to the formation of said primary or secondary precipitations, since nitrogen shall not exist in the steel above impurity level or as an accessory element from the manufacture of the steel, i.e. max 0.3%, normally max 0.1%.
Vanadium can partly be replaced by niobium up to max 2% niobium, but this opportunity is preferably not utilised. Typically, the said hard particles to the great part consist of MC-carbides, more particularly substantially V4C3-carbides. The said hard particles are comparatively large and it is estimated that at least 50 vol-% of the hard particles exist as finally dispersed, discrete particles in the matrix, having sizes between 3 and 20 xcexcm. Typically, they have a more or less rounded shape. These conditions contribute to the provision of a good hot-workability of the steel. Furthermore, because of the high hardness of the hard particles of said MX-type, and because of the sizes of the particles, they also to a great degree contribute to the provision of a desired abrasive wear resistance of the material.
The vanadium content shall be at least 6.5% and max 15% and preferably max 13%. According to one aspect of the invention, the vanadium content is max 11%. According to another aspect of the invention, the vanadium content preferably shall be at least 7.5% at the same time as the maximum vanadium content amounts to 9%. According to still another aspect of the invention, the preferably chosen vanadium content, however, shall lie between 6.5 and 7.5%. When it is here referred to vanadium it shall be recognised that vanadium completely or partly can be replaced by twice the amount of niobium up to max 2% niobium.
The carbon content shall be adapted to the content of vanadium and any existing niobium in order that there shall be obtained 10-40 vol-%, and according to some, above mentioned aspects of the invention, more particularly 10-25 vol-% or 20-40 vol-% of said primarily precipitated hard particles of MX-type, and also 0.6-0.8, preferably 0.64-0.675% carbon in the tempered martensite, wherein also the fact shall be considered that secondary precipitation of in the first place MC-carbides and M7C3-carbides can occur to some extent, said secondary precipitation also consuming some carbon. The conditions that apply for the relations between vanadium and niobium on one side and carbon on the other side are visualised in FIG. 2, which shows the carbon content versus the content of V+2 Nb. In the co-ordinate system in FIG. 2, where the content of V+2 Nb is abscissa, and the carbon content forms the axes of ordinates, the corner-points of the drawn figures have the co-ordinates stated in Table 3.
According to a first aspect of the invention, the contents of vanadium, niobium, carbon+nitrogen shall be adapted to each other such that the said co-ordinates will lie within the range of the area defined by the comer-points A, Bxe2x80x3, E, F, Bxe2x80x2, B, C, D, A.
According to a second aspect of the invention, the contents of vanadium, niobium, carbon+nitrogen shall be adapted to each other such that the said co-ordinates will lie within the range of the area defined by the comer-points A, B, C, D, A.
According to a third aspect of the invention, the contents of vanadium, niobium, carbon+nitrogen shall be adapted to each other such that the said co-ordinates will lie within the range of the area defined by the corner-points A, Bxe2x80x2, Cxe2x80x2, D, A in the co-ordinate system in FIG. 2.
According to a fourth aspect of the invention, the co-ordinates shall lie within the range of the area defined by the corner-points A, Bxe2x80x3, Cxe2x80x3, D, A.
According to a fifth aspect of the invention, the co-ordinates shall lie within the range of the area defined by the corner-points A, Bxe2x80x3, Cxe2x80x2xe2x80x3, Dxe2x80x2, A.
According to a preferred embodiment, the co-ordinates preferably may lie within the range of the area defined by the corner-points A, Bxe2x80x2, Cxe2x80x2, Cxe2x80x3, Cxe2x80x2xe2x80x3, Dxe2x80x2, A.
According to another preferred embodiment, the co-ordinates preferably may lie within the range of the area defined by the corner-points Bxe2x80x3, Bxe2x80x2, Cxe2x80x2, Cxe2x80x3, Bxe2x80x3.
According to a still another preferred embodiment, the co-ordinates lie within the range of the area defined by the comer-points Dxe2x80x2, Cxe2x80x2xe2x80x3, Cxe2x80x3, D, Dxe2x80x2.
The above mentioned second through fifth aspects, and said preferred embodiments, particularly concern the use of the steel for cold work tools. According to a sixth aspect of the invention, which particularly concerns the use of the steel for tools or machine parts for working ceramic masses, e.g. within the brick industry, the contents of vanadium, niobium and carbon+nitrogen may be adapted to each other such that the co-ordinates of said points will lie within the range of the area defined by the corner-points E, F, Bxe2x80x2, Bxe2x80x3, E in the co-ordinate system in FIG. 2.
According to a seventh aspect of the invention, the co-ordinates more particularly may lie within the range of the area defined by the corner-points E, F, Fxe2x80x2, Exe2x80x2, E.
According to an eighth aspect of the invention, the co-ordinates should lie within the range of the area defined by the comer-points Exe2x80x2, Fxe2x80x2, Fxe2x80x3, Exe2x80x3, Exe2x80x2, and according to still another aspect within the range of the area defined by the comer-points Exe2x80x3, Fxe2x80x3, Bxe2x80x2, Bxe2x80x3, Exe2x80x3.
Chromium shall exist in a amount of at least 5.6%, preferably at least 6%, suitably at least 6.5%, in order that the steel shall get a good hardenability, i.e. an ability to be through-hardened also in case of thick steel objects. The upper limit of possible content of chromium is determined by the risk of formation of non-desired M7C3 carbides because of segregation during the solidification of the melt. The chromium content therefore must not exceed 8.5% and should preferably be less than 8%, suitably max 7.5%. An amount of 7% is a typical chromium content, which is comparatively low in view of the desired hardenability.
In order that the material nevertheless shall get desired hardenabilty, without risk of serious segregation, the steel alloy also shall contain at least 1.7% molybdenum, preferably 1.7-3% molybenum, suitably 2.1-2.8 molybdenum. Typically, the steel contains 2.3% molybdenum. Molybdenum in principle completely or partly may be replaced by the double amount of tungsten. Preferably, however, the steel does not contain tungsten more than at impurity level.
Silicon and manganese may exist in amounts which are normal for tool steels. Each of them therefore exists in the steel in amounts between 0.1 and 2%, preferably in amounts between 0.2 and 1.0%. The balance is iron and impurities and accessory elements in normal amounts, wherein the term accessory elements means harmless elements which normally are added in connection with the manufacture of the steel and which may exist as residual elements.
The following is a conceivable, preferred composition of the steel according to the invention: 2.55 C, 0.5-1.0 Si, 0.5-1.0 Mn, 7.0 Cr, 8.0 V, 2.3 Mo, balance iron and unavoidable impurities and accessory elements.
Another conceivable, preferred composition is: 2.7 C, 0.5-1.0 Si, 0.5-1.0 Mn, 7.0 Cr, 8.0 V, 2.3 Mo, balance iron and unavoidable impurities and accessory elements.
Still another conceivable, preferred composition is: 2.45 C, 0.5-1.0 Si, 0.5-1.0 Mn, 7.5 Cr, 8.0 V, 2.3 Mo, balance iron and unavoidable impurities and accessory elements.
The above mentioned conceivable, preferred compositions of the steel of the invention are particularly suited for cold work steels. A conceive, preferred composition for the use of the steel for tools and machine parts for working cheramic masses is: 3.5 C, 0.5-1.0 Si, 0.5-1.0 Mn, 7.0 Cr, 12.0 V, 2.3 Mo, balance iron and unavoidable impurities and accessory elements.
Another conceivable, preferred composition for said use is: 3.9 C, 0.5-1.0 Si, 0.5-1.0 Mn, 7.0 Cr, 14.0 V, 2.3 Mo, balance iron and unavoidable impurities and accessory elements.
Still another conceivable, preferred composition for said use is: 3.0 C, 0.5-1.0 Si, 0.5-1.0 Mn, 7.0 Cr, 10.0 V, 2.3 Mo, balance iron and unavoidable impurities and accessory elements.
At the manufacture of the steel material of the invention there is first produced a melt having the characteristic, chemical composition of the invention. This melt is cast to ingots or castings, wherein the melt is caused to solidify so slowly that there is precipitated in the melt during the solidification process 10-40 vol.-%, preferably, depending on the intended use of the steel, 10-25 vol.-% or 20-40 vol.-% of hard particles of MX type, where M is vanadium and/or niobium, preferably vanadium, and X is carbon and nitrogen, preferably essentially carbon, at least 50 vol.-% of said hard particles having sizes between 3 and 20 xcexcm, and that the material, in connection with the heat treatment of the steel material, possibly after hot working and/or machining to desired product shape, is heated to a temperature within the temperature range of 900-1150xc2x0 C., where the micro-structure of the steel alloy at equilibrium consists of austenite and hard particles of said MX type, that the material is maintained at this temperature for a period of time of 15 min-2 h, from which temperature the material is cooled to room temperature, wherein the austenitic matrix of the steel is transferred to martensite containing said primarily precipitated hard particles and carbon in solid solution, and that the material subsequently is tempered once or several times at a temperature of 150-650xc2x0 C.
Further characteristic features and aspects of the invention and advantages and effects that can be achieved through the invention will be apparent from the appending patent claims and from the following description of performed experiments and calculations.